1. Field of the Invention
The present invention provides a new method of seismic prospecting usable in particular for offshore operations.
2. Description of the Prior Art
Seismic prospecting at sea is generally achieved by towing behind a ship a seismic source adapted for intermittently emitting seismic pulses and a reception assembly or seismic streamer adapted for picking up the acoustic signals reflected or refracted by the different layers of the subsoil in response to the acoustic pulses. It is general practice to tow the source-streamer assembly as if these two elements were rigidly joined together (except for the lateral drift of the streamer).
The signals picked up are sent to a central recording facility on the ship and seismic sections are formed from the recordings of the profiles, after effecting processing and various corrections.
A conventional processing method consists in forming multiple plots of the different points of the horizon or surfaces reflecting the seismic waves.
A seismic streamer comprises a number N of receivers each formed by a sensor or more generally by a plurality of interconnected sensors whose combined signals form a distinct plot on recording. The N receivers are spaced apart along the seismic streamer with a constant spacing e.
With the seismic streamer and the source towed along the profile to be explored, at each of the positions of a succession of different positions of the profile a seismic emission and reception cycle is carried out and for each of them N distinct plots are recorded. Then, from the recorded plots obtained after the succession of cycles, those plots are selected which correspond to reflections from identical mirror points. After correction of the travel time differences of the waves due to differences in slant of the acoustic rays, these plots are combined so as to obtain by a redundancy effect a greater sharpness in the representation of the different reflecting surfaces.
The maximum multiple coverage order; i.e. the maximum number of plots which it is possible to combine, is equal to the number N of receivers in the streamer. It can only be attained if, from one firing to the next, the emission and reception device has only advanced a distance equal to half the spacing e between receivers, because the emission points and the corresponding reception points, relative to fixed reflecting points, move apart symmetrically from each other, a movement of the emission points causing an equal movement, but in the opposite direction, of the reception points.
For the same reflecting point, the distance which separates the firing point and sensor increases then from one firing to another by twice the distance which the source-streamer assembly has covered during each emission cycle. If the number N is too great, this cannot be achieved in practice because of the time required for recording all the useful echoes and because of the technical and economic constraints which preclude sailing below a certain minimum speed of a few knots. Reflecting points spaced apart by an interval e/2 may be restored, but with a smaller degree of coverage.
The seismic prospecting method of the invention increases very substantially the multiple coverage order which it is possible to obtain with a given streamer or maintains it at a high level while using one or more much shorter streamers.